Cable systems utilize transmission lines to transfer high speed signals such as video signals. Example transmission lines include coaxial cables and twisted pairs of wires, and example video signals include analog and digital video signals. In a typical cable system, a cable driver is used to drive the transmission line so that signals can be properly interpreted by the receiving system.
Coaxial cables typically have a nominal line impedance of either fifty ohms (50Ω) or seventy-five ohms (75Ω), while twisted pairs of transmission lines typically have a nominal line impedance of about one-hundred ohms (100Ω). Each transmission line needs to be terminated with another impedance that is matched in value to the nominal line impedance (e.g., 50 Ω, 75Ω or 100Ω). Failure to provide a properly valued terminating impedance may result in signal reflections from the receiver end of the transmission line back to the driver at the other end of the transmission line. Signal reflections may also occur when there is a break in the transmission line or a required ground connection is missing.
For one example driver, a two (2) volt peak-to-peak signal is driven onto a transmission line with a characteristic impedance of 75Ω. When properly terminated with an impedance of 75Ω, the total impedance seen by the driver looking into the transmission line is 150Ω. At the receiving end of the transmission line, the signal level is ideally one (1) volt peak-to-peak. However, when the terminating impedance is missing or the signal ground is broken at the receiving end, the voltage at the receiving end will be up to two (2) volts peak-to-peak, which may fluctuate due to signal reflections from the high impedance of the open circuit condition. When the terminating impedance is incorrect (e.g., 25Ω) for the 75Ω line, the voltage will be approximately half the expected amplitude lower, which may also fluctuate due to signal reflections from the mismatched impedance.